Two-stage chemical mixing system

Abstract

A chemical mixing system for mixing ultra pure water and concentrated HF to a desired concentration within a qualification range. Batches of the mixed chemical are mixed in a relatively small mix vessel and transferred to a relatively large mix drum. The concentration of the chemical within the mix drum is measured by a concentration monitor. If the measured concentration of the chemical in the mix drum is within the qualification range, the batches of chemical in the mix vessel are mixed to the desired concentration. If the measured concentration of the chemical in the mix drum is below the qualification range, the batches of chemical in the mix vessel are mixed to a high concentration which is greater than the desired concentration. If the measured concentration of the chemical in the mix drum is above the qualification range, the batches of chemical in the mix vessel are mixed to a low concentration which is less than the desired concentration. Batch averaging is thereby performed to maintain the concentration of chemical in the mix drum within the desired qualification range.

What is claimed is: 
     
       1. A chemical mixing system for mixing at least first and second chemical constituents to obtain mixed chemical having a desired concentration within a qualification range, including: a first constituent inlet for receiving a first chemical constituent;   a second constituent inlet for receiving a second chemical constituent;   a mix vessel for mixing mix vessel batches of the first and second chemical constituent;   a first line for fluidly coupling the first constituent inlet to the mix vessel, and including a first line valve for controlling the flow of the first chemical constituent through the first line;   a second line for fluidly coupling the second constituent inlet to the mix vessel, and including a second line valve for controlling the flow of the second chemical constituent through the second line;   first mix vessel level sensing means for providing first vessel level signals when the mix vessel is filled to a first level, wherein the first level corresponds approximately to the volume of the first chemical constituent that will provide mix vessel batches having the desired concentration;   second mix vessel level sensing means for providing second vessel level signals when the mix vessel is filled to a second level, wherein the second level is a level greater than the first level by an amount which corresponds approximately to the volume of the second chemical constituent that will provide mix vessel batches having the desired concentration;   third mix vessel level sensing means for providing third vessel level signals when the mix vessel is filled to a third level which is greater than the first level and less than the second level;   fourth mix vessel level sensing means for providing fourth vessel level signals when the mix vessel is filled to a fourth level which is greater than the second level;   a mix drum for holding and mixing a plurality of mix vessel batches of the mixed chemical;   a mix drum line for fluidly coupling the mix vessel to the mix drum, and including a drum line valve for controlling the flow of the mixed chemical through the mix drum line;   a concentration monitor for providing concentration signals representative of the concentration of mixed chemical within the mix drum; and   a control system coupled to the first line, second line and drum line valves, the first second, third and fourth mix vessel level sensing means and the concentration monitor, for controlling the mixing of the chemical constituents in the mix vessel and the transfer of the mix vessel batches to the mix drum, including: first control means for actuating the first line valve to fill the mix vessel to the first level with the first chemical constituent;   second control means for actuating the second line valve to fill the mix vessel from the first level to the second level with the second chemical constituent if the concentration of the blended chemical within the mix drum is within the qualification range;   third control means for actuating the second line valve to fill the mix vessel from the first level to the third level with the second chemical constituent if the concentration of the blended chemical within the mix drum is greater than the qualification range;   fourth control means for actuating the second line valve to fill the mix vessel from the first level to the fourth level with the second chemical constituent if the concentration of the blended chemical within the mix drum is less than the qualification range; and   fifth control means for actuating the drum line valve to transfer the mix vessel batch of mixed chemical to the mix tank.     
     
     
       2. The chemical mixing system of claim 1 wherein: the system further includes high mix drum level sensing means for providing high drum level signals when the mix drum is filled to a relatively high level; and   the control system is coupled to the high mix drum level sensing means and further includes sixth control means for causing mix vessel batches to be mixed and transferred to the mix drum only if the level of mixed chemical within the mix drum is less than the relatively high level.   
     
     
       3. The chemical mixing system of claim 2 wherein: the system further includes low mix drum level sensing means for providing low drum level signals when the mix drum is filled to a relatively low level; and   the control system is coupled to the low mix drum level sensing means and further includes means for actuating the second line valve to fill the mix vessel from the first level to the second level with the second chemical constituent, and not the third or fourth levels, when mixing mix vessel batches if the level of blended chemical in the mix drum is less than the relatively low level.   
     
     
       4. The chemical mixing system of claim 1 wherein: the system further includes low drum level sensing means for providing low drum level signals when the mix drum is filled to a relatively low level; and   the digital control system is coupled to the low mix drum level sensing means and further includes means for actuating the second line valve to fill the mix vessel from the first level to the second level with the second chemical constituent, and not the third or fourth levels, when mixing mix vessel batches if the level of blended chemical in the mix drum is less than the relatively low level.   
     
     
       5. The chemical mixing system of claim 1 and further including a vacuum system for motivating the first and second chemical constituents into the mix vessel. 
     
     
       6. The chemical mixing system of claim 5 and further including a pressure system for motivating the mix vessel batches from the mix vessel to the mix drum. 
     
     
       7. The chemical mixing system of claim 6 and further including a pressure system for motivating the mixed chemical from the mix drum. 
     
     
       8. The chemical mixing system of claim 1 and further including: a recirculation line for recirculating mixed chemical in the mix drum; and   motivating means for motivating chemical from the mix drum through the recirculation line.   
     
     
       9. The chemical mixing system of claim 8 and further including means for mounting the concentration monitor in the recirculation line. 
     
     
       10. The chemical mixing system of claim 1 wherein the mix vessel is smaller than the mix drum.